This invention pertains to a process for the preparation of low molecular weight methacrylate polymers and copolymers and particularly to the free radical polymerization of methacrylate monomers and lesser amounts of other ethylenically unsaturated monomers in the presence of a dioxime pyridine complex of Cobalt(II). The process is particularly useful for controlling molecular weights to produce low molecular weight methacrylate polymers and copolymers.
Current techniques for controlling molecular weights of polymers involve the addition of chain transfer agents, such as mercaptans, to the polymerization media. The occurrence of chain transfer to the chain transfer agent results in decreased molecular weight of the polymer formed. However, the addition of chain transfer agents can cause several problems including consumption of the chain transfer agent resulting in broad molecular weight distributions, odor problems related to the chain transfer agent and increased cost of the final product due to the chain transfer agent. A recent publication (Enikolopyan et al., J. Poly. Sci., Polm. Chem. Ed., 19, 879 1981), described the use of cobalt porphyrin complexes as catalysts for chain transfer to monomer. Each porphyrin molecule causes up to 2,000 transfer reactions compared to chain transfer agents which typically are active in only one transfer reaction. A recent journal article, "Acc. Chem. Res 16, 343 (1983) discloses the use of cobalt dimethylglyoxime complexes as catalysts in free radical reactions although polymerization reactions are not disclosed.
It now has been found that the use of very low amounts of a dimethylglyoxime (DMG) or other dioxime pyridine complexes of Cobalt(II) provides excellent chain transfer mechanisms for controlling polymer molecular weight to produce low molecular weight methacrylate polymers and copolymers. Low molecular polymer weight between about 500 and 20,000 containing between about 5 and 200 average copolymerized monomer units can be produced by polymerizing methacrylate monomers with minor amounts of other ethylenic monomers if desired in the presence of 0.001% by weight dimethylglyoxime pyridine Co(II) in combination with azo polymerization catalysts. Polymer synthesized in accordance with this invention exhibit controlled low molecular weight on the order of 500-20,000 as compared to about 100,000 in the absence of the Cobalt(II) transfer agent. The process is particularly suitable for controlling the molecular weight of methacrylate polymers and copolymers. A further advantage of the Co(II) complex is that polymers produced exhibit a much improved color. The Co(II) glyoxime is also lower cost than the porphyrin based catalysts. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.